The present invention relates to error detection in digital equipment, and more particularly to error detection for digital television equipment that assures error detection when only the active picture portion of a television signal is transmitted.
Television signals, both audio and video, are now represented and processed digitally. This leads to a different class of signal distortion from the analog world. Analog distortions, such as rolloff, are now yielding to data bit errors in current digital equipment. Whereas rolloff problems usually start off subtly, equivalent to least-significant bit errors in a digital domain, digital errors can affect with equal likelihood most-significant and least-significant bits. Moderate analog domain errors are by their nature relatively innocuous, while moderate digital domain errors, left uncorrected, are not.
Even a robust serial digital interface between equipment can introduce errors. The interface is particularly vulnerable to excessive cable run length due to poor installation, impaired coax cable due to broken shield braids, accidental damage and the like, or aging or failing components in the receiver and transmitter VCO circuits or equalizers, for example. Many of these potential problems show up as intermittent errors, perhaps imperceptible on a studio monitor. These errors visibly or audibly degrade the signal as the fault grows less intermittent.
The Society of Motion Picture and Television Engineers (SMPTE) has proposed a serial digital interface standard that is extremely robust. An automated mechanism whereby single- and multiple-bit errors are detected by studio equipment in service has been proposed as a recommended practice by SMPTE. The detection occurs before the error rate becomes visually or audibly perceptible, and the presence of detected errors is reported to either studio equipment and/or studio operators for corrective action. The error detecting mechanism has a transmitter generating a set of check words, such as parity, checksum and cyclic redundancy check (CRC), that are sent to a receiver. The receiving equipment compares the received check words with its own internally generated set of check words.
Three different error detection mechanisms are proposed, one for each of three classes of transmitted data. Ancillary data, which include audio and teletext data, are checked against the parity bits and checksum words, required by the SMPTE serial standard, that are transmitted with the data. Active video and full field data are checked by sixteen-bit CRC words. CRC words are generated once per video field both in the transmitting equipment and the receiving equipment. At the beginning of the following field the CRC words are transmitted, as ancillary data, and the receiver compares the received CRC words with its self-generated CRC words. A discrepancy denotes a data error.
Active picture checking covers the data samples that represent the active video portion of a video signal. Full field checking covers all data including ancillary data, vertical blanking interval data and horizontal blanking interval data. This is an all-encompassing error check, designed to test all transmitted data. The only data not covered by full field checking are three lines during the vertical interval. At the beginning of these three lines an error data packet is inserted. The second of the three lines is where many switching devices make switches, and the last of these lines is to allow settling time after any switches. As shown in FIG. 1 a full field of video data includes vertical sync and blanking intervals (or a non-video data area for component video), horizontal intervals and an active picture (AP) area. The full field (FF) CRC is generated from the first data word, fff, on the line following the three-line switching interval and ends with the last data word, eef, on the line in the next field preceding the three-line switching interval. The active picture CRC is generated from the first data word, aaa, within the active picture area through the last data word, eea, within the active picture area, excluding all data words in the horizontal intervals. Both CRCs are inserted in the next field within the horizontal interval of the first line of the three-line switching interval as part of the error data packet. The CRCs are sixteen bits of data calculated using the CRC-CCITT polynomial generation method: EQU CRC=X.sup.16 +X.sup.12 +X.sup.5 +1.
Generally, the sixteen bits of data are placed in two or more eight-bit order samples.
However certain video equipment strips the vertical sync and blanking intervals and the horizontal intervals from the video data so that only the active picture area is processed. This results in loss of the full field and active picture CRCs that are located within the vertical/horizontal intervals. Thus there is no way to verify data errors for such equipment.
What is desired is error detection for digital television equipment that indicates data errors solely within the active picture area of a video signal where the proposed SMPTE recommended practice is not available.